The invention relates to a semiconductor rectifier, and in particular to a semiconductor rectifier using a trench structure.
A wide variety of different semiconductor rectifiers are known. The most widely adopted are conventional bipolar diodes and Schottky diodes.
A particularly desirable rectifier parameter is a good blocking characteristic in the off-state, including a high breakdown voltage. It would accordingly be desirable to provide a rectifier with such enhanced blocking capability.
One method of providing a rectifier is to use a conventional MOSFET as a rectifier. In this case, in the on state the channel is essentially resistive giving some conduction even at very low forward voltages. The disadvantage is that the gate of the MOSFET needs to be controlled synchronously with the voltage across the source and drain of the MOSFET in order to provide rectifying action. This requires additional control circuitry and is generally undesirable.
There thus remains a requirement for an improved rectifier.
According to the invention, there is provided a semiconductor rectifier, comprising: an anode contact and a cathode contact; an intermediate semiconductor region of first conductivity type extending between the anode and the cathode contacts; at least one trenched gate electrode flanking the intermediate semiconductor region; a shield semiconductor region of second conductivity type opposite to the first conductivity type between the at least one trenched electrode and the intermediate semiconductor region, the shield semiconductor region being connected to one of the anode and the cathode contacts; a gate insulating layer between the shield semiconductor region and the trenched electrode; and a separate gate contact for applying a voltage to the gate to deplete the intermediate semiconductor region; wherein the intermediate semiconductor region can be depleted by applying a voltage to the gate and a voltage of a first polarity between anode and cathode and when a voltage of opposite polarity to the first polarity is applied between anode and cathode the shield semiconductor region conducts to shield the intermediate semiconductor region from the voltage applied to the gate to allow a conductive path through the intermediate semiconductor region between the anode and the cathode.
The invention works by applying a constant voltage to the gate sufficient to deplete the intermediate region. However, the shield region is filled with carriers when a voltage of one polarity is applied between the anode and cathode. For example, with an n-type intermediate region and a p-type shield region connected to the anode, the p-type shield regions will be filled with holes when the anode is positive with respect to the cathode. In this state, the shield regions will shield the intermediate regions from the bulk of the effect of the applied gate voltage, so that the anode and cathode are connected through the intermediate semiconductor region. The on-resistance of this structure can be quite low.
Keeping to the same example, when the anode is negative with respect to the cathode, the shield regions will be depleted and no longer shield out the effect of voltage applied to the gate electrode. The gate electrodes are then free to deplete the intermediate region and thus prevent a current flow between anode and cathode.
Preferably, the anode contact to the intermediate semiconductor region is a Schottky contact. Using such a structure, the Schottky contact causes rectification at low anode-cathode voltages. At higher forward voltages, the device conducts. At higher reverse voltages, the shield semiconductor region is depleted, as is the intermediate semiconductor region, and some of the voltage is dropped across the intermediate region. In this way, higher breakdown voltages can be obtained than simply using the Schottky contact itself for any given doping of the intermediate region. Preferably, the Schottky contact is a low barrier Schottky to minimise voltage drops in the structure. Preliminary modelling results suggest that the device according to the invention may deliver a greater improvement in breakdown voltage than using a trenched Schottky device where the gate is connected to the anode rather than to a more negative constant voltage source as in the present invention.
Conveniently, the intermediate semiconductor region may be a semiconductor body having opposed first and second faces, with the anode contact on one face and the cathode on the opposite face. The trenched gate electrode may then be provided in an insulating trench extending through the body towards the second face from the first face. The wall and base of the trench may be insulated by the gate insulating layer.
The shield semiconductor region may then extend along the outside of the side walls of the trench.
In an alternative structure, the shield semiconductor region may extend not merely along the side walls of the trench, but also underneath the base of the trench. In this way, the effect of any regions of high ionisation integral and hence high risk of breakdown that might otherwise be present at the corner between the intermediate semiconductor region, the shield semiconductor region and the gate insulating layer can be reduced. The alternative arrangement accordingly can provide a higher breakdown voltage.
The most similar device of which the applicants are aware is a static induction thyristor (SIT). This relies on depletion layers spreading from buried p-type regions to join up and prevent current flow between anode and cathode. Accordingly in an SIT the p-regions will deplete the path between anode and cathode regardless of the voltage between anode and cathode. The arrangement of the invention works in a similar but different way. Again, a depletion region extends to pinch off conduction but it will do this depending on the voltage applied to the p-region. The polarity of the voltage applied between anode and cathode causes the shield regions to conduct or not to conduct, and hence allows or prevents conduction between anode and cathode.
The structure according to the present invention responds spontaneously to changes in the anode bias to go into a conducting or a blocking state depending on the polarity. Thus, the conducting and blocking states come about automatically when the anode and cathode are biased, without the need to drive the gate xe2x80x9csynchronouslyxe2x80x9d with changes in anode biased plurality, as would be required using a MOSFET.
The only additional circuitry required is a simple negative voltage source to permanently bias the gate. This is available in many application circuits and does not present a significant drawback.
It will be appreciated the invention could be applied to either p-type or n-type circuits, in a variety of different materials, dopings or structures.
Other advantageous technical features of the present invention are set out in the attached dependent claims.